CDMA techniques transmit multiple information signals on the same channel, and differentiate each user sub-channel by encoding it with a unique spreading code. Originally, CDMA networks were designed to carry only voice traffic and exhibited limited data rate variability. CDMA networks, however, are evolving to encompass a variety of applications, each requiring potentially different data rates or quality of service needs. A CDMA system that employs four spreading codes is said to have a “spreading factor” of four and can transmit four distinct streams of data.
The output of a given CDMA transmitter is generally the sum, s, of each data stream multiplied by the corresponding spreading code. In a system having a spreading factor of four, there are 16 possible output vectors, s. It can be shown that the peak-to-average power ratio of such a CDMA system is the maximum value over each possible output vector, s, of the peak energy divided by the average energy. If the peak-to-average power ratio of a CDMA system is greater than one then it is obvious that power of different signals vary. Thus, when the output vector, s, is amplified prior to transmission by a non-linear amplifier, which are commonly found in communications devices, the transmitted signal will be amplified differently for each possible output vector, s, creating non-linear distortion.
A need therefore exists for a CDMA system that provides a reduced peak to average power ratio.